1. Field of the Invention
The present invention relates to a position detection apparatus for detecting the surface position of an object, an exposure apparatus having the position detection apparatus, a control method therefor, and a device manufacturing method.
The position detection apparatus of the present invention can be applied to, e.g., a surface position measuring apparatus for measuring a small distance by an electrostatic scheme. The exposure apparatus of the present invention can be applied to, e.g., a slit-scan exposure apparatus.
2. Description of the Related Art
As surface position detection apparatuses for semiconductor exposure apparatuses, position detection apparatuses which obliquely irradiate a semiconductor wafer, placed at a position where a mask pattern is to be transferred through a projecting lens, with light from a projector, and detect light obliquely reflected by the surface of the semiconductor wafer to detect the surface position are widely used. FIG. 9 is a schematic view of such a conventional surface position detection apparatus.
As shown in FIG. 9, illumination light emitted from the output end of an optical fiber 9 illuminates a pattern forming plate 11 through a condenser lens 10. The illumination light passing through the pattern forming plate 11 is projected onto the exposure surface of a wafer 5 through a lens 12, mirror 13, and projection objective lens 14, so the image of the pattern on the pattern forming plate 11 is formed on the exposure surface of the wafer 5 from an oblique direction with respect to an optical axis EX. The illumination light reflected by the wafer 5 is reprojected onto the light-receiving surface of a light-receiving device 18 through a condenser objective lens 15, mirror 16, and imaging lens 17. The image of the pattern on the pattern forming plate 11 is formed on the light-receiving surface of the light-receiving device 18. When the wafer 5 moves in the vertical direction, the image of the pattern moves to the left or right on the light-receiving surface 18. When an arithmetic circuit 19 calculates the position of the pattern, the surface position of the wafer 5 can be detected.
In the exposure apparatus, such a focus detection system has a plurality of measurement points in one shot (area to be exposed) of a wafer. A surface position Z and tilt component (tilt) of the measurement shot are calculated by comparing the measurement results at the plurality of measurement points within the X-Y plane. Highly accurate focus position control is achieved by controlling a Z·tilt stage 8.
In recent years, a slit-scan exposure apparatus which exposes while holding a reticle and wafer conjugate with a projecting lens and scanning both the reticle and wafer to increase the exposure area has received a great deal of attention. In this exposure apparatus, a focus detection signal is directly used as a closed loop signal for controlling the posture of the stage. As the signal, a signal as smooth as possible, i.e., a signal averaged for an exposure area is necessary.
However, since the slit light projection scheme cannot measure position while uniformly illuminating a measurement surface of interest, the measurement areas always become discrete. In addition, experiments conducted by the present inventors have revealed that when a thin slit-like light beam irradiates the edge portion of a step, the reflected light is scattered to generate a large focus detection error.
This problem can be solved by using an electrostatic sensor as a focus detection sensor. An electrostatic sensor is more advantageous as a focus detection sensor of a slit-scan exposure apparatus than an optical sensor because it can almost uniformly average within the detection area, does not generate any focus detection error at an edge portion, and has a high response speed.
FIG. 10 is a view showing the principle of distance measurement by an electrostatic sensor. Referring to FIG. 10, a flat electrode 30 used for measurement is arranged near an object 31 to be measured. A high-frequency voltage is applied from an oscillator OS to the flat electrode 30. An ammeter AM is connected between the flat electrode 30 and oscillator OS. The ammeter AM and a measurement device (circuit) 32 connected to the ammeter AM measure the magnitude of an AC current flowing to the flat electrode 30. The current measurement result is input to an arithmetic circuit 33. A distance do between the flat electrode 30 and the object 31 to be measured is measured by arithmetic processing by the arithmetic circuit 33.
Although the above-described surface position detection apparatus can obtain high accuracy in detecting the surface position of a substrate having a chip layout (pattern of a chip) compatible with the apparatus, no high accuracy can be obtained in detecting the surface position of a substrate having a chip layout incompatible with the apparatus.
In addition, when the shape of the wafer surface changes to a shape incompatible with the apparatus because of the repeated lithography process, the above-described surface position detection apparatus cannot accurately detect the surface position of the substrate.